Temporal stability of polymorphic Arctic charr parasite communities reflects sustained divergent trophic niches

Abstract Polymorphic Arctic charr Salvelinus alpinus populations frequently display distinct differences in habitat use, diet, and parasite communities. Changes to the relative species densities and composition of the wider fish community have the potential to alter the habitat niche of sympatric Arctic charr populations. This study evaluated the temporal stability of the parasite community, diet, and stable isotopes (δ13C, δ15N) of three sympatric Arctic charr morphs (piscivore, benthivore, and planktivore) from Loch Rannoch, Scotland, in relation to changes to the fish community. All Arctic charr morphs displayed distinct differences in parasite communities, diet, and stable isotope signatures over time, despite the establishment of four new trophically transmitted parasite taxa, and increased fish and zooplankton consumption by the piscivorous and planktivore morphs, respectively. Native parasite prevalence also increased in all Arctic charr morphs. Overall, Loch Rannoch polymorphic Arctic charr morph populations have maintained their distinct trophic niches and parasite communities through time despite changes in the fish community. This result indicates that re‐stocking a native fish species has the potential to induce shifts in the parasite community and diet of Arctic charr morphs.


| INTRODUC TI ON
Freshwater systems are the most threatened ecosystems on earth (Albert et al., 2021;Kernan et al., 2011;Reid et al., 2019;Woodward et al., 2010). Furthermore, human-induced global change has placed severe pressure on aquatic ecosystem structure and function, and poses a considerable threat to biodiversity (e.g., Christensen et al., 2006;Ficke et al., 2007;Jackson et al., 2016) and food webs (Nagelkerken et al., 2020;Petchey et al., 1999). Model predictions suggest the geographic distributions of many fish species, including Arctic charr (Salvelinus alpinus L.), will be significantly reduced as a result of climate warming (Chu et al., 2005;Hein et al., 2012;Svenning et al., 2022). Moreover, given the importance of biological interactions as drivers of ecological processes (Blois et al., 2013), freshwater fish communities have the potential to be highly affected by several other commonly conducted anthropogenic activities. For instance, fish re-stocking and fish introduction can impact the native fish community (Britton et al., 2011) by modifying inter-and intraspecific competition for food, habitat, and niche space between fish species (e.g., Britton et al., 2010;Gregersen et al., 2006;Klemetsen et al., 2003), parasite component communities (i.e., parasites found in a host population, here in a morph; e.g., "spillback" impact on native fishes; Kelly et al., 2009) and changes in predator-prey relationships (L' Abée-Lund et al., 1992). Arctic charr is the world's northernmost freshwater fish species  and thus represents an interesting model species to study the influences in multiple anthropogenetic stressors occurring at the southern edge of this species' geographic distribution.
Arctic charr express high levels of phenotypic plasticity, with up to five different morphs known to co-occur in a single water body (Doenz et al., 2019;Skúlason et al., 1989), and may also form genetically segregated populations (Moccetti et al., 2019;Praebel et al., 2016;Simonsen et al., 2017;Verspoor et al., 2010). Sympatric Arctic charr morphs can be distinguished by their size, head morphology, and stable isotope tracers (δ 15 N and δ 13 C), the stable isotope value of which depends on foraging habits (Doenz et al., 2019) and trophic niche (e.g., Adams et al., 2003). Commonly, morphs segregate along the benthic-pelagic resource axes where one morph feeds in the benthic environment (benthivore morph) and one morph relies more heavily on food available in the water column (planktivore morph; Adams, 1998;Skúlason et al., 1989;Walker et al., 1988).
Arctic charr are known to host more than 40 metazoan parasites (Moravec, 2004). The diversity of their parasite community is attributed to their diet since most known parasite taxa of Arctic charr are trophically transmitted via the consumption of intermediate hosts, often invertebrates or small fish (Moravec, 2004). Thus, Arctic charr morphs are exposed to different parasite communities depending on habitat choice, feeding habits, and the presence of the intermediate hosts in their occupied habitat (Frandsen et al., 1989;Jonsson & Jonsson, 2001;Knudsen et al., 1997;Sandlund et al., 1992).
In this context, trophically transmitted parasite communities can reveal the temporal stability of the food web (Behnke et al., 2018), since their complex life cycles span multiple trophic levels.
In this study, we evaluated the temporal stability of the parasite infracommunity of a polymorphic Arctic charr population in Loch Rannoch following brown trout Salmo trutta (L.) re-stocking and crucian carp Carassius carassius (L.) introduction (see Fraser & Adams, 1997). A risk exists that introduced fish species bring alien generalist parasites that successfully establish in native fish (e.g., Asian fish tapeworms and yellow grub; Dove et al., 1997;Gaglio et al., 2016;Kuchta et al., 2018), although most parasites tend to be specialized in one or few types of host. While brown trout were already part of the native community, trout originating from a different lake/hatchery are potentially exposed to different parasites, and thus may introduce novel parasite taxa to the system (i.e., a translocation impact: Kelly et al., 2009;Peeler et al., 2011). Arctic charr and brown trout share many parasite species that can be translocated along with the host and establish in the local fish community (e.g., Adolfsen et al., 2021;Bristow, 1993;Knudsen et al., 2007). The parasite load in the system can be indirectly amplified (Kelly et al., 2009) or diluted (Goedknegt et al., 2016) as higher salmonid density can act as an enlarged pool of hosts for native parasites. In addition, the brown trout and crucian carp are two benthivore fish and they could compete for common resources with Arctic charr (Eloranta et al., 2013;Langeland et al., 1991). It is also possible that introduced fishes cause indirect changes in the parasite community of Artic charr through modifying predator-prey links that expose hosts to a different range of parasites or by acting as parasite sinks themselves, thus reducing Arctic charr exposure (Poulin & Mouillot, 2003).
Overall, the Arctic charr parasite community in Loch Rannoch might change through time due to the introduced-relocated benthivore competitor (e.g., brown trout and crucian carp) or other possible explanations (i.e., annual variability). This increase in benthivore fishes might promote the copepods transmitted parasite (Dorucu, 1996;Dorucu, Adams, et al., 1995). However, the parasite community of the three morphs of Arctic charr might still be different if their diet and habitat are stably diverged through time (Dorucu, 1996). Indeed, we expect that Arctic charr morphs maintain trophic niche partitioning. Thus, we hypothesized that: (a) trophic niches of the three Arctic charr morphs will be stable through time (i.e., diet and isotope); thereby (b) the parasite component communities in Arctic charr morphs will remain distinctly different between morphs as all the taxa previously recorded are trophically transmitted. These two hypotheses are assessed in this study using a contemporary snapshot of the trophic information provided by diet analysis and the proxies of longer-term trophic niche provided by stable isotope analyses (δ 13 C, δ 15 N) and parasite communities.
Arctic charr were sampled using gill nets in October 1992 and July 1993 (n = 253; see Dorucu, Adams, et al. (1995), Dorucu (1996) for details), and in October 2010 (this study, n = 101). Gill nets were deployed overnight for a maximum period of 12 h during both study periods. Between 1992 and 93, 30 benthivore, 173 planktivore, and 50 piscivore Arctic charr were collected. In 2010, 34 benthivore, 34 planktivore and 33 piscivore Arctic charr were sampled (Table 1). All fish were frozen and transported to the Scottish Centre for Ecology and the Natural Environment (SCENE), University of Glasgow for subsequent analysis . Fork length (mm) was measured for all sampled fish.

| Diet analyses
The stomach fullness was determined from the dissection of the alimentary canal. Stomach contents collected from the upper end of the esophagus to the pyloric sphincter were identified to the lowest practical taxonomic level (typically order or family) under a stereomicroscope. The diet groups identified in 1992-93 (Dorucu, 1996) served as a reference point for the analyses in 2010 (zooplankton, copepods, surface insects, chironomid larvae, Pisidium, insect larvae, Gammarus, unidentified invertebrates, fish). The frequency of occurrence of each prey category was evaluated as volume percentage for each stomach and each food category (Hyslop, 1980). Schoener's index (Wallace Jr, 1981) was used as a proxy for diet overlap between different morphs in each time period and the two sampling periods for each morph. This index is usually considered as biologically meaningful when its value exceeds 60% (Wallace Jr, 1981). For comparison, the frequency of occurrences of each prey in 1992-93 was extracted from Dorucu (1996) using DataThief III software (Tummers, 2006).

| Parasitological analyses
Parasite prevalence (i.e., proportion of host individuals of an Arctic charr morph that were infected) and mean abundance (i.e., the mean number of parasites in a given host morph) were calculated for each parasite species (Bush et al., 1997). In our study, the parasite prevalence data for Arctic charr from 1992-93 were obtained from Dorucu (1996) using DataThief III software (Tummers, 2006). The fish sampled in 2010 were examined for metazoan parasites using a stereomicroscope, with parasites morphologically identified to species or genera using taxonomical criteria (e.g., Moravec, 2004) before specimens were fixed in absolute ethanol for molecular analyses. We selected some of the specimens used for the morphological analyses and rehydrated them, as preservation in absolute ethanol shrink and/or modify the internal structures of the worms. We prepared whole mounts according to Bray (2010) andJustine et al. (2012) protocols. Unfortunately, the preservation quality of the specimens (e.g., poor quality tubes and wrong concentration of alcohol) did not allow a more accurate morphological identification, and molecular identification was needed.
Molecular data were obtained from a subsample of specimens for each prospective parasite taxa (from 2010 only) to confirm their morphological identification. DNA was extracted using Chelex® in deionized water containing 0.1 mg/ml proteinase K. A partial fragment of the large ribosomal subunit (28S rDNA) was chosen as a marker because it is broadly used to molecularly assign parasitic flatworms and acanthocephalans to known genera/species (Blasco-Costa et al., 2016) and a partial fragment of the small ribosomal subunit (18 S rDNA) was amplified for the nematodes since it is the most common marker used for this group (Černotíková et al., 2011). The PCR amplification protocol for the 28 S marker followed Blasco-Costa et al. (2009), for the 18 S followed Černotíková et al. (2011) and for the COI followed Blasco-Costa et al. (2014). Purified amplicons were sent to Macrogen Europe (Amsterdam, Netherlands) for sequencing from both strands, with the same PCR primers used for amplification.
Sequences were assembled and inspected for errors using Geneious® ver. 8.1.9 (Kearse et al., 2012) and submitted to GenBank® (accession numbers in Table S1). Available sequences for taxa belonging to the same family/genus/species as our presumed taxa were obtained from GenBank® and aligned with our sequences to validate species identification or improve the preliminary identification based on specimen morphology. Following this, alignments were obtained using the default parameters in MAFFT (Katoh et al., 2005) and were trimmed at their extremities.
Parasite phylogenetic reconstructions were carried out using maximum likelihood (ML) and Bayesian inference (BI) criteria. The model of nucleotide evolution GTR (general time-reversible model) with a gamma distribution using among-site rate variation (Γ) was applied to all analyses. ML analyses were conducted using RAxML TA B L E 1 Arctic charr sampled in Loch Rannoch (1992-93 from Dorucu, Adams, et al. (1995) and 2010).

| Stable isotope analyses
Dorsal muscle tissue samples from 32 benthivore, 32 planktivore, and 21 piscivore Arctic charr sampled in 2010 were dried at 50°C for 24 h, before being ground to a fine powder with a mortar and pestle, and weighed for analysis (0.3 mg). Carbon (δ 13 C) and nitrogen (δ 15 N) isotopes were then analyzed from individual fish at the University isotope ratios were expressed in standard delta notation (‰) relative to the international reference materials of Vienna PeeDee Belemnite for carbon (Craig, 1957) and atmospheric nitrogen (Mariotti, 1983).
Data quality control was monitored, and corrections were made using a mix of international and in-house standards (e.g., cellulose and bovine liver) cross-calibrated against International Atomic Energy Agency standards for Carbon (CH3, CH6) and nitrogen (N1, N2).
No <20% of the samples included in any run consisted of standards and reference materials, with obtained measurements used in data normalization and to ensure measurement precision and accuracy.
Associated QC/QA checks indicated an error for reportable data of no more than 0.2‰ and 0.3‰, respectively, for δ 13 C and δ 15 N.

| Statistical analyses
All the analyses were computed with the statistical software R version 4.1.0 (www.r-proje ct.org). Separate generalized linear models (GLM) were used to investigate the influence of fish morph (benthivore, planktivore, piscivore) and fish length on species richness, total parasite taxa abundance, and the abundance of each parasite taxon among the three Arctic charr morphs (2010 only). Models were fitted with appropriate Poisson or quasi-Poisson distributions, to account for over-dispersion (see Tables S2 and S3) detected by AER::dispersiontest (Kleiber & Zeileis, 2008). Additionally, the correlation of variables was assessed using the function corPlot::psych (Revelle, 2011) and cor.test::stat (R Core Team, 2021), and interactions between variables were included in the GLM models when taxa were correlated with another variable (i.e., fish length in our model).
Dissimilarity among the parasite infracommunities (i.e., parasite taxa abundance of an individual fish) in 2010 as a function of Arctic charr morph (Krebs, 1999) was assessed with nonmetric multidimensional scaling (NMDS) analyses using the zero-adjusted, Bray-Curtis dissimilarity measure, which is not affected by the number of null values between samples (Clarke et al., 2006). To account for species absences in some infracommunities (zero-inflated data) a "dummy species" was added to all communities (see Clarke et al. (2006)). The significance of the predictor variables (i.e., fish morph), was tested with the Adonis function. These analyses were conducted with the package vegan (Oksanen, 2020) and MASS (Venables & Ripley, 2002).

Multivariate analysis of variance tests (MANOVA) and Wilcoxon
tests were used to decide whether the distributions of δ 13 C (reflecting the origin of the carbon in the tissue of the fish) and δ 15 N (reflecting its trophic position) values among the three Arctic charr morphs differed (2010 only). The isotopic niche overlaps were also assessed between Arctic charr morphs using a Bayesian approach derived from Swanson et al. (2015) implemented in the package nicheROVER (Lysy et al., 2014). This method provides the 95% probability and credibility interval that one individual from one morph could be found within the niche of another morph.
In addition to the species above, four taxa were recorded for the

| Parasite component communities among Arctic charr morphs
In addition to the presence of four previously undetected parasite taxa in Arctic charr, the prevalence of parasites increased in each morph between 1992-93 and 2010 (see Figure 1b and Table 2).
The total prevalence of parasite infection was high in the fish sampled in 2010, 100% of the piscivore and planktivore morphs and 88.2% of the benthivore morph. The piscivore morph showed a significantly higher overall parasite abundance than the benthivore morph, which also showed a higher overall parasite abundance than the planktivore morph ( Table 2). The three morphs showed distinctive patterns of infection with differences in parasite prevalence ( Figure 1b and Table 2). The eye flukes (Diplostomum spp. and Tylodelphys sp.) and the two nematode taxa (P. salvelini and the unknown nematode) were more common in benthivore morph (see Table 2). The cestode taxa, Dibothriocephalus spp. and E. salvelini were more common in the planktivores (94 and 68%, respectively) than in the piscivores (88 and 51%) or benthivores (21 and 45%, respectively). Proteocephalus longicollis was more common in the planktivores (36%) than in the other morphs. The three remaining taxa (C. pseudofarionis, E. truttae, and N. rutili) were mainly recovered from piscivorous Arctic charr (63% versus <23% in two other morphs for C. pseudofarionis and 21% versus <4% in the planktivores for the acanthocephalans). Finally, the two acanthocephalans and the unknown nematode were relatively uncommon (prevalence 0%-21%) in all morphs. Moreover, our results also showed that the overall parasite species richness did not vary between the morphs, but the total abundance of parasites increased with fish length (Table S2).

| Parasite infracommunities
A clear segregation between the parasite communities of the three Arctic charr morphs was found, despite some overlap between the planktivore and piscivore morphs (nonmetric multidimensional scaling analysis; Figure 3). The analysis had a stress value of 0.14, which fell within the accepted range (<0.2; Clarke et al., 2006). Additionally, these were supported by a significant difference (Adonis test; pvalue = .001) in the parasite species composition and abundance in the infracommunities between the three morphs. The numerical vectors in the plot show that parasite infracommunities of the benthivore morph were mainly composed of nematodes (P. salvelini and an unknown nematode) and the GLM also showed a higher abundance of P. salvelini in this morph in contrast with the two other morphs, especially when compared to the planktivores that show the lowest abundance (t value = −2.6 and p-value = .011; Table S3).

| Stable isotope analysis
Our study was able to evaluate the temporal stability of polymorphic

| DISCUSS ION
Loch Rannoch Arctic charr morphs displayed high trophic niche stability over time. This is reflected in the consistent divergence in the parasite infracommunities and component communities' structure, diet composition, and stable isotope analyses of the three sympatric F I G U R E 3 Nonmetrical multidimensional scaling biplot based on Bray-Curtis dissimilarity among parasite infracommunities (N = 91). Infracommunities are color-coded according to the host morph (greenbenthivore morph; blue-piscivore morph; pink-planktivore morph). Ellipses regrouped 95% the parasite infracommunities of a particular host morph (colored accordingly). The vectors with arrows in black indicate the contribution of each parasite taxa to the dissimilarity. Random jitter (0.1) was added to the plot to improve visualization of overlapping data points. Asterisks represent p-values lower than .001 for the Adonis test results.

F I G U R E 4
Plot of the stable isotopes δ 13 C and δ 15 N and probabilistic niche overlap calculated using the 95% niche regions between each pair of Arctic charr morphs from Loch Rannoch (2010). Colors refer to fish morph: green-benthivore morph; blue-piscivore morph; and pinkplanktivore morph. Ellipses regrouped 95% of the fish of a particular morph (colored accordingly). Mean probability (%) (range 95% credibility interval) of finding an individual of the morph in the row within the niche region of the morph in the column.
morphs. Overall, an increase in the prevalence (%) of all native parasite taxa in all three sympatric Arctic charr morphs was recorded between 1992-93 and 2010. Four novel macroparasite taxa were found in 2010, C. pseudofarionis, P. longicollis, P. salvelini, and an unidentified nematode. These parasites are generalist species commonly found as adults in salmonids, (e.g., Chubb, 1963;Moravec, 2004), thus they could have been introduced by the re-stocking of brown trout. The introduction of crucian carp may not be linked to the presence of these new taxa, as the only parasites are known to infect both crucian carp and Arctic charr are Diplostomum spathaceum (Rudolphi, 1819), Acanthocephalus lucii (Müller, 1776), and N. rutili (Karvonen et al., 2005). Moreover, it is unlikely that these parasite taxa were missed during the first study from Dorucu, Adams, et al. (1995), as the sampling in 1992-93 was larger than in 2010 and the relatively high prevalence of these new parasite taxa.
In our study, the piscivore morph had a low diet overlap with both the benthivore and planktivore morphs, which is consistent with earlier trophic studies in Loch Rannoch Dorucu, 1996), and with other studies on similar charr-morph pairs (Knudsen, Gjelland, et al., 2016;Moccetti et al., 2019). The low diet overlap is supported by the distinct parasite infracommunity composition observed between the two sampling periods , and the markedly different stable isotope values for all morphs from 2010. The planktivore morph had a high diet overlap with the benthivore morph, mainly due to the high consumption of chironomid larvae by both morphs, and the relatively low consumption of zooplankton and copepods by the planktivore morph. The unexpected low zooplankton diet of the planktivore morph contrasted with earlier trophic studies in Loch Rannoch that showed >90% zooplankton in the planktivore diet (Dorucu, 1996;Walker et al., 1988). The difference may relate to the pooling of fish dietary data from October and July in the earlier study Dorucu, 1996) and the use of October-only data in 2010. For example, seasonal abundances of zooplankton in Scottish lochs vary, being higher in June and July than in October (Romo, 1990). However, the high diet overlap between the planktivore and the benthivore morphs did not reflect their long-term niche divergence, as was indicated by timeintegrated trophic tracers used here in the form of the parasite community and stable isotopes (mainly δ 15 N values) analyses in 2010 that suggest highly diverged dietary niches over a period of several months. Altogether, our results confirm that the trophic niches are divergent between the sympatric morphs and seem to have remained relatively stable through time, i.e., over the 18 years between the two sampling periods used in this study. Similar trophic stability has been reported in other studies of lakes containing two and three sympatric morphs of Arctic charr (e.g., Knudsen et al., 1997;Knudsen et al., 2014;Siwertsson et al., 2016).
The planktivore morph was mainly infected with parasites transmitted by copepods, such as P. longicollis, Dibothriocephalus spp. and E. salvelini, as was noted in the earlier study of these morphs (Dorucu, Adams, et al., 1995). These parasite taxa are typically associated with other planktivore morphs of Arctic charr (Frandsen et al., 1989;Moccetti et al., 2019) and other salmonid species (Chubb, 1982;Knudsen et al., 2003). In addition to possible seasonal differences in availability, the relatively low occurrence of zooplankton prey in the planktivore morph diet could be linked to an introduced competitor of the planktivores such as juveniles crucian carp that feed on plankton (Penttinen & Holopainen, 1992), and juvenile native perch population that have increased in abundance in recent years (C.E. Adams personal com.). Moreover, Loch Rannoch is a relatively small water body and Arctic charr is a highly mobile species. Thus, the possibility of inter-specific interactions remains despite the potentially low abundance of crucian carp. Additionally, the sampling methods (i.e., time in the gillnets) can impact the occurrence of zooplankton prey in the planktivore morph, as zooplankton digestion will continue as long as the fish is alive in the gillnet. By contrast, infections of P. longicollis and Dibothriocephalus spp. were low in the benthivore morph (6 and 21%, respectively) suggesting a low consumption of zooplankton by this morph over time. The prevalent infection of C. pseudofarionis trematodes (23%) and P. salvelini nematode (39%) transmitted by insect larvae or amphipods and oligochaetes, respectively, were expected because the benthivore morph diet is apparently dominated by benthic prey.
The parasite community of benthivore morphs is mainly composed of taxa that use benthic species as intermediate hosts, such as snails, insect larvae, and amphipods (Knudsen et al., 2014). The divergence of the δ 15 N stable isotope signals noted in this study corroborates the dissimilar parasite communities and the diet differences of the benthivore and planktivore morphs.
There has been a general increase in infection levels of the na- morphs (up to 94%). Both these taxa can be transmitted to the fish host either by feeding on copepods directly or through feeding on fish prey (with parasite re-establishment in the predator). The increase in prevalence suggests altered transmission rates resulting from either change in the zooplankton community and/or the prey fish community. Re-establishment from prey fish explains the very high infection of these two parasite species in the piscivore morph in 2010, which were feeding intensively on fish.
The newly recorded parasite taxa (i.e., C. pseudofarionis, P. salvelini, P. longicollis, and the unknown nematode) in 2010 should be regarded as commonly occurring (prevalence up to 73%). These parasite taxa are easy to identify due to their distinct shape and size, and were unlikely to have been missed in the earlier 1992-3 study. Proteocephalus longicollis and C. pseudofarionis are common and relatively abundant in many salmonids, as well as in brown trout and Arctic charr (Moravec, 2004;Scholz & Hanzelova, 1998;Soldánová et al., 2017). Their presence is most likely attributed to the stocking of brown trout in the lake in previous years, with three of the new parasite species (i.e., C. pseudofarionis, P. salvelini, P. longicollis) known to infect different brown trout populations in the UK (Hartvigsen & Kennedy, 1993;Kennedy, 1978). None of these potentially introduced parasite species are regarded as problematic for Loch Rannoch Arctic charr. Crepidostomum pseudofarionis has not been previously reported as pathogenic for its final fish host (Moravec, 2004) and P. longicollis is usually not or only slightly pathogenic (Bauer et al., 1977;Moravec, 2001a;Scholz, 1999).
Additionally, although capillariids are generally considered to be pathogenic at high infection levels, P. salvelini have not previously been reported as problematic (Moravec, 2001b;Moravec, 2004) and their abundances were generally low even in the most infected Arctic charr morph. Fraser & Adams, 1997) by fisherman and the increase in benthivore competitors (e.g., brown trout and perch), which may have altered the inter-specific competition between fish species and changed predator-prey relationships (e.g., Britton et al., 2010;Gregersen et al., 2006;Klemetsen et al., 2003), and thereby changed transmission rates and routes of parasites to the three Arctic charr morphs. As the parasite communities of crucian carp are very different from salmonids (Karvonen et al., 2005), it is unlikely that crucian carp and Arctic charr share parasite taxa. Introductions of fish or crustaceans have in many cases been shown to change lake ecosystems in the United Kingdom (Adams, 1994;Adams & Mitchell, 1992) but have also changed interactions between native sympatric species/morphs (Taylor et al., 2006) including Arctic charr morphs . Indeed, if the population of benthivore (e.g., crucian carp, brown trout, and perch) increases in future, it could affect the benthivore and planktivore morphs of Arctic charr. For instance, the introduced crucian carp predominantly feed on chironomid larvae and benthic cladocerans (among other benthic invertebrates) and could potentially compete with Arctic charr Fraser & Adams, 1997;Penttinen & Holopainen, 1992). However, the introduction of crucian carp in the system should not be the main vector of the change in the parasite community of Arctic charr as they are not abundant in the lake (C.E. Adams pers.com). Alterations in the native fish community (e.g., trout stocking or increases in the perch population) may also initiate cascades in the food-web structure and Arctic charr niche (Sandlund et al., 2016). Then, the changes in the fish community might subsequently directly and indirectly change the parasite community structure of native (i.e., Arctic charr) fish hosts (e. g. Amundsen et al., 2013;Kelly et al., 2009;Kuhn et al., 2015). This change in the parasite community has also likely happened in the fish community of Loch Rannoch.
Overall, the Arctic charr populations in Loch Rannoch have lower infection levels of helminths compared with other northern lake systems Kuhn et al., 2015;Paterson et al., 2018).
Unfortunately, the intensity of parasite infections from the earlier study on Loch Rannoch system was not reported by Dorucu, Adams, et al. (1995), thereby preventing an assessment of the evolution of the parasite load through time. However, even if none of the potentially introduced species (i.e., C. pseudofarionis, P. salvelini, P. longicollis) are considered as highly detrimental to Arctic charr, the increase in infection of the native Dibothriocephalus spp. and E.
Given that Loch Rannoch supports the only known population of a piscivore morph of Arctic charr in Scotland Gardner et al., 1988;Walker et al., 1988), an increased parasite load along with additional anthropogenic-induced stressors (e.g., fishing activity) could pose threats to the unique Arctic charr populations supported by this lake (Adams, 1994(Adams, , 1996Fraser & Adams, 1997).
In conclusion, the habitat, and the trophic behavior of the Arctic

ACK N OWLED G M ENTS
We want to thank Derek Pretswell for assistance during the fishing process, and Karin Strand Johannessen and Karolyn O'Connor for assistance in the laboratory. We are thankful to Amy Ellison for technical help in the molecular work. We are grateful to Gabrielle Grenier and André Frainer for their comments and advice on an early version of this manuscript. We thank the Loch Rannoch Conservation Association for their support and permission to collect charr in Loch Rannoch.

FU N D I N G I N FO R M ATI O N
The genetic analyses of Diplostomum were funded by the Cardiff

CO N FLI C T O F I NTE R E S T
None declared.

O PE N R E S E A RCH BA D G E S
This article has earned an Open Data badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/ dryad.jdfn2 z3f2.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data from the manuscript is publically available in the Dryad database (https://doi.org/10.5061/dryad.jdfn2 z3f2). The DNA sequences will be submitted to Genbank (https://www.ncbi.nlm.nih. gov/genba nk/).